The present invention relates to monitoring the ground circuit in an electrical power distribution system of a type that is commonly employed at a location where coal is mined from the surface: i.e., at a strip mine. More particularly, this invention is an improvement over the invention disclosed in U.S. patent application Ser. No. 937,680 entitled Ground Monitoring System, in the name of John R. Sherwood, the disclosure of which is incorporated herein by reference.
In surface mining operations, extensive use is made of large electrically powered machinery such as power shovels, pumps, drills, etc. Accordingly, 600 volt to 25 kilovolt three phase electrical service may be provided at a remote surface mining site. Electrical utility company power lines are brought to a substation at or close to the mining site. Because much of the mining machinery must be movable, long insulated power cables that can be dragged along the ground connect the machinery to mobile switch houses that in turn are connected to the substation by additional long insulated power cables. There are a number of different types of power cables used in surface mining. One such type of cable includes the three power conductors, each with a metallic shield about it, two bare ground conductors, and an insulated ground check wire that commonly is called a pilot wire. All are enclosed in an insulating elastomeric jacket. Typically, the cables are in 1,000 feet lengths. Two or more cables commonly are connected together to extend the length of the distribution system.
If an insulation breakdown occurs in one of the large electrically powered pieces of machinery the entire machine may be at a dangerous high voltage unless it is protected by a suitable metallic ground circuit. In many instances the machine will be in contact with the earth, but because the machinery is moved around from place to place, a dependable earth ground resulting only from contact with the earth cannot be relied upon. Consequently, a metallic ground circuit in the power cable is essential. The Federal Coal Mine Health and Safety Act of 1969 requires that the electrical ground circuit in the power cable be continuously monitored to detect short circuits, open circuits, and abnormally high resistance conditions in the ground circuit. Upon detection of a fault condition the ground monitor must open one or more circuit breakers to disconnect the electrical source from the faulty portion of the distribution system.
A ground monitor intended for use at a surface mining location must be able to detect the condition that it is the metallic ground conductor of the power cable that is providing ground continuity in the system and not an earth ground resulting from the machine being in contact with the earth or water.
In the above mentioned Sherwood application, a test signal is coupled from a fault sensor circuit onto a ground monitoring loop in each length of cable. The loop includes the ground wires and pilot wire of the cable. The apparatus continuously monitors the integrity of the ground and pilot wires of the cable and produces a fault signal upon the occurrence of a short circuit, an open circuit, or high resistance that exceeds a predetermined normal value that is included as part of the loop. It is absolutely necessary to the proper operation of the apparatus of that system that the test signals propagate over the ground wire in the cable and not through earth ground. It is obvious that if the test signal can return to its source through earth ground as well as through the ground conductors of the cable it may be impossible to reliably monitor the conditions of the ground conductor. In the Sherwood application, an inductor that presents a large impedance to the test signal but not to the 60 Hz power current is connected in series with the ground conductors at the end of each length of cable. This large impedance blocks the test signal and prevents it from leaving the test loop of the respective length of cable.
This system operates satisfactorily, but does have several limitations. First, the large inductor connected in series with the ground conductors does add some resistance to the ground wires. Safety codes and regulations limit the amount of resistance that is permitted in the metallic ground circuit of the electrical distribution system at a surface mining site. With the inductor series connected with the ground wire of the cable, the number of cables that may be connected together in series without exceeding the permitted resistance value is limited to approximately five lengths of cable. At some mining locations more than five lengths of series connected cables are required. Consequently, the above system may not be adequate for all applications.
Furthermore, the fact that the inductor is series connected with the ground conductors represents a potential limiting factor in the integrity of the ground conductors of series connected cables. It is fundamental that the more components and connections that are placed in a series circuit, the greater the chances are that a fault will occur in the circuit.